1. Field of the Invention
The present invention is concerned with radioactive well logging and, more particularly, logging operations that are performed in a nonfluid-filled borehole.
2. Description of the Prior Art
Commonly, radioactive well logging in cased, air or gas-filled boreholes requires either special equipment or, at a minimum, extensive corrections to the log produced by a sonde intended for use under fluid-filled borehole standard conditions. Generally, the magnitude of the correction required increases with the inner diameter of the borehole. The change from a fluid-filled to a substantially air-filled borehole, however, is frequently such a significant departure from the design conditions of the sonde that correction of the log with acceptable accuracy is usually not possible, regardless of the borehole inner diameter.
It is desirable, therefore, that the sonde be compensated for the lack of fluid in the borehole, so that the combined result of other borehole variables, such as temperature and pressure, requires only a small correction to the log.
Prior art well logging systems have used displacement sleeves in fluid-filled, cased-hole radioactive logging. When chlorine logging in large, cased-holes using a neutron source, for example, it is desirable to use a displacement sleeve when the casing is filled with salt water. The sleeve serves the purpose of minimizing, to the greatest extent possible, the effects of the neutron capture by the chlorine in the borehole fluid by physically displacing the fluid located between the sonde and the casing in the source/detector area. Such fluid displacement sleeves are generally formed to slide over a sonde and are clamped or otherwise held in position to displace borehole fluid from around the sonde to the outside diameter of the sleeve, which is less than the casing inside diameter by an amount adequate to provide the required sleeve/casing clearance and borehole fluid bypass space.
Also known, is the use of a sleeve on the outside of a sonde to shield the sonde from certain types of radioactive emissions. It may be desirable, for example, to shield a sonde having a detector that measures neutron capture gamma rays of a selected energy range from all arriving neutrons in order to limit the effects on detected gamma ray count rates of neutron capture by sonde materials. Such a shielding sleeve would be manufactured of a material containing a good neutron absorber, such as boron carbide, in sufficient quantity to absorb any expected incidence of neutron irradiation. Commonly, both fluid displacement and shielding functions are required of a single sleeve, and assignee of the present invention has had such sleeves in commercial use for a number of years. However, such shielding sleeves have not been directed to the problem of compensating a sonde for a lack of fluid in a borehole.